fix(lxc): preserve credentials and IP across reinstall

Three related fixes so login/password/SSH key/IP survive a reinstall:

1. backend/app/api/proxmox/lxc.py — on initial LXC deploy save
   cloud_init_user='root' and cloud_init_password=req.password to VMInstance
   (previously both were None, so reinstall could not restore root password).

2. backend/app/api/proxmox/async_ops.py — _do_reinstall_sync passes
   cached.cloud_init_password to create_lxc_container so the new container
   gets the same root password as the original.

3. backend/app/workers/monitoring_worker.py — VM sync worker no longer
   overwrites VMInstance.ip_address with NULL when no IPAM allocation
   exists for that (server_id, vmid). It only updates the field when
   IPAM has a value, otherwise the IP saved at deploy/wizard time is
   preserved.

backend/app/api/proxmox/lxc.py

@@ -280,8 +280,8 @@ def _do_lxc_deploy(task_id: int, req: LXCDeployRequest, user_id: int, username:
             ip_address=allocated_ip,
             ip_prefix=int(network_mask),
             gateway=gateway,
-            cloud_init_user=None,
-            cloud_init_password=None,
+            cloud_init_user="root",
+            cloud_init_password=req.password,
             ssh_keys=req.ssh_keys,
             template_id=None,
             template_name=req.ostemplate,